Transmission circuits



Aug. 3 1926.,

' A. B. CLARK ET AL TRANSMISSION CIRCUITS Filed Oct. 24, 1925 [ch0 Swap/visor IN VEN TORS MORNE Y Patented Aug. 3, 1926.

UNITED STATES PATENT ALVA B. CLARK, OF BROOKLYN,

NEW JERSEY, nssienons a coaroaarron or NEW TO YORK.

OFFICE.

TRANSMISSION cmcmrs.

Applicationflled October 24, 1925. Serial Nb. 64,654.

V This invention relates to tems, and more particularly to transmission systransmission sytems adapted for interconnecting public address systems or radio tions or the like, to enable a widely separated audiences to broadcastlng staplurality of hear speech others,

difficulty arises form the fact that the audience at a particular station will hear the program from the local transmitter better than programs transmitted from distant transmitters. This arises from the fact that a transmission loss inevitably occurs in transmitting from the distant to a articular local station. Heretofore, it has posed to provide a en prospecial interconnecting arrangement between each receiver and its local transmitter to regulate the volume transmitted, but such arrangements are not satis actory from distant fects. It is the present invention to provide rangement for interconnecting where the transmission loss points is determined by echo eftherefore'one of the objects of a circuit ara plurality of public address systems or radio broad- 0 casting stations in such a manner that a local audience will hear the program from the local transmitter with substantially the same volume as thatfrom a d ist-ant' transmitter notwithstagding echo efi'ects. 'Anothor object. of the invention is arrangement by which the mission circuits may be through provision will be made fore to provide an veral transinterconnected a common station at which suitable qnalizingthe 4 transmission to all of the stations and for overcoming echo effects.

These and other objects of the invention means of the-arrangements are attained by set forth in the following de'ta iled description and illustrated in the accompanying drawings. Figures 1 and 2 of tratetwo forms of circuit arran bo ymg the levea wn,

which illusg lllfillts em- Before proceeding to a description of the explanation is desirable of is involved in interconnectpublic address systems or invention, some the problem that ing two or more broadcasting stations. If we assume a transmission line interconnecting two distant points, and at each station a terminal arrangement comprising a transmitter con volume of sound emitted by the loud speaker at a given station will be local transmitter is operated than when the distant transmitter is operated. This difference between the transmission will be equal to the transmission loss in the line.

Where the minimum net loss inthe transmission line is not. limited by talker echo effects, the terminal stations of the lines may be arranged so that the transmitting and receiving branch are interconnected with the line through a hybrid coil and balancing network, as shown at 10 in Fig. 1. The local transmitter and receiver may be made substantially conjugate with respect to each other by this arrangement so that the local transmitter does not transmit to the local receiver through the common connection to the line. If then a byass connection, including an amplifier B is provided, the transmission between the transmitter and the loud speaker may be determined by the amplifier BAf- In the absence of echo effacts, the amplifier BA, may be adjusted to render the transmission from the local transmitter to the local receiver equal to that from the distant transmitter to the 10- cal receiver.

ar on the equalization of volume, it is desirable to say a word as to the nature of echoes. In' general echoes'are of two kinds -talker echoes and listener echoes. When the voice, currents originating at'a transmitts? a e rensn t d greater when the In order to understand how echo'efl'ects to adistant point.

' the two-way repeaters in each raised one unit. Obviously, the direct transand there encounter impedance irregularities, the currents reflected back to the local receiver are known as talker echo currents. If, however,'the voice currents, after being reflected back once encounter another impedance irregularity "and are again reflected in the direction of original transmission, the currents thus transmitted to the distant receiver are known as listener echoes.

Now suppose we have a transmission line with a number of two-Way repeaters located at suitable points, and the gain of one of direction is mission over the line is only raised one unit. Talker echoes, however, pass through this repeater twice, once in the direction of original transmission and once when they are refiected back to the local receiver. Listener echoes, on the other hand, will pass through the repeater three times, for not only are they reflected back towards the local receiver, but they are again reflected through the repeater in the original direction. viously, then, the echo currents are amplified to a greater extent than the direct transmission, and as We transmission loss of the line by increasing the gain of the amplifiers, a point is reached where the echo currents have been so amplified as compared with the direct transmission that the situation becomes intolerable. Thus, it'will be seen that echo currents become a limiting factor in determining the transmission equivalent of the curcuit, and limit the gain to which the repeaters may be set to a point below singing Depending on the line, its physical make-up, etc., in some instances talker echoes limit the gain of the repeaters in the line and in other instances listener echoes limit the gain. 0

When the line connecting two stations s of such type that its minimum net loss is limited by talker echoes, the circuit arrangement just described does not permit the equalization of volumes because the hybrid coil does not keep the echoes from the line out of the receiver. The'talker echoes coming from the'line will be heard in the re ceiver with the same volume as if the transmitter and receiver branch were directly connected to the line in parallel without the bypass arrangement. Consequently, the bypass amplifier BA, must be adjusted to give at least thesame volume of sound from the transmitter as in the case where the transmitter and receiver are directly connected to the line. That this is so will be clear if we consider that the amplifier BA, adjusted so that the same volume of sound is heard in the receiver R, from the local transmitter as would be the case where the transmit- I ter and receiver circuits are directly connected to the line. If, under these condiattempt to reduce the the electrical characteristics of tions, the amplifiers in the assumed transmission lines to connect two distant stations are adjusted until talker echoes limit further increase in the gain of the repeaters, it will be evident that the gain of the amplifier BA: can not be cut down without reducing the signal as compared with the echoes. As this would he intolerable under the conditions just outlined, it is clear .that the difference between the volumes of sound from the local transmitter and the distant transmitter is the same in both arrangements under the conditions where the minimum net loss is limited by talker echoes.

it the assumed transmission line is of such character that its minimum net los is determined by listener echoes or (b other causes than talker echoes) it would be possible to reduce the gain of the by-pass amplifier such as BA or increase the "ain of the receivlng amplifier such as R t until talker echoes prevent further changes. If there were no talker echoes present the arrangements employing the hybrid coil connection and the bypass might be adjusted to pro duce a full equivalence between signals in the local transmitter and signals from the distant transmitter. present,-hoWe'Ver, the equalizing adjustment can notproceed beyond the point at which the talker echoes efi'ectively limit the gain. In the foregoing discussion no considera- If talker echoes are tion has been given to an additional difiiculty which arises where more than two distant stations are interconnected. If we interconnect any considerable number'of widely separated stations in such a manner as to furnish two-way transmission between any giyen station and all of the others, the transmission loss between different pairs of stations is in general different, so that the lietenerat onestation does not receivethe same volume from all of the other stations. The arrangements which are now about to be described provide means for overcoming not only' the difficulty just stated but'the difficulties due to echoes. To accomplish the latter result use is made of echo suppressors which eliminate or greatly reduce the echoes to secure a complete equalization. The transmission losses between stations can therefore be reduced and the listener enabled to hear the distant stations more loudly than would otherwise be possible. The equalization of the transmission loss between stations is accomplished by using the echo suppressors in combination with an interconnecting arrangement whereby a plurality of lines may be associated with each other for between a station associated transm ssion with any lineand the stations of all the other lines. This interconnecting arrangement in the case of Fig. 1 may be a star-shaped systemof lines radiating. from a common point at the each branch of the star'for equalizing transmission. In the case of Fig. 2 the interconnecting arrangement may comprise a multlple-way repeater, each repeater unit of which is provided with an echo suppressor.

Referring to Fig, 1, L L,, L and L designate transmission lines of any type extending from an interconnecting point F to distant stationsB, C, D and E (not illustrated) These distant stat-ions may be provided with transmitting and receiving. equipment of any type. TlllS equipment might be the ordinary to comprising a. high quality transmitter, loud speaking receiver and circuit connections in volving a hybrid coil, balancing network and by-pass as already described. If a talking and listening station is to be provided central point F, it may be connected In by means of the hybrid coil 10, one-way am lifiers TA, and RA and by-pass ampli er BA, arranged as already described.

A multiple-way artificial line built up of resistances is used to form the connection. This line is like the ordinary H-type of artificial line except that there is a pair of series resistances Z Z etc., for each of the several stations to be connected. These series resistances are all equal in value. There is also a shunt resistance Z, common to all the con nections. By properly proportioning these resistances the multiple-way line can be made to present to each repeater, such as R R E and R the impedance with which it is designed to work. These repeaters which are merely shown conventionally, except in the case of R are all alike and have th A network introduces the same loss btween any given repeater and any other repeater. The function of the repeater is of course, to adjust the transmission between the line with which it is associated and the star connection so that the-over-all transmission from the terminal station to the star connection will be the same for all of the lines involved. It will be apparent from the nature of the star network that there is a certain minimum loss for which the network may be designed and built, this loss becoming greater as the number of branches increases.

The repeater lt is shown in detail and comprises the usual amplifier AW for transmitting from east to west, and amplifier AE for transmitting from west'to east, the amplifiers being provided with potentiometers as is usual for adjusting the gain. The amplifiers are connected to the terminal line L through a hybrid coil 20, a balancin network 21 of the usual type being provi ed to render the repeater branches con: jugate. The repeater branches are also connected to the correspondin branch Z of the star netwark through a hy rid coil 22. The

ephone substation set, or a sete same impedance. The star I rents impedance of the star network may"be accurately balanced by means of a simple resistance 23.

The repeaters R R etc., schematically indicated, are similar to the repeater R above described. The gain of each repeater is set so that equal net losses occur between each distant terminal station and the ter-' minals of the artificial line. The net loss between any two stations is then the same and any station hears all of. the other stations with e ual volume. In the case of the local transmitter and receiver at the station the volume is determined by the amplifiers TA, and RA, so that no special repeater asjust described is necessary.

hen transmission takes place from any station it enters the star network and goes out to all the other stations. Echoes then return from each of the other stations to the central point from which point the echoes pass through all of the stations including the transmitting station, so that the echoes may be heard by all stations. To prevent these echoes an echo suppressor is applied to each 22-repeater such as the repeater R This echo suppressor is schematically indicated and may be of a well known type including a suitable amplifier and rectifier for amplifying and rectifiying voice currents and'a relay for controlling a short circuit contact, the relay being operated by the rectified voice currents. As shown, the input of the echo suppressor S is connected to the midpoints of the hybrid coil 22, and the short-circuiting contacts are connected across the midpoints of the hybrid coil 20. the impedance balance between the artificial line and the balancing resistance 23 is good enough to prevent false operation due to unbalanced cur- When transmission originates at the station D, the echo suppressor S will not operate by currents ori 'najting at the station D and hence, there wi 1 be no necessity to provide a corresponding echo suppressor unit connected in the opposite direction andy arranged to disable the echo suppressor unit 8. The echo suppressor unit S is so connected that when transmission takes place from the interconnecting point out toward the station D, echoes from the terminal stations are prevented from coming back over the line L to the common point. In other words, talking currents coming from the star network tothe repeater R operate the echo suppressor unit F 3. and short circuit the midpoints of the hybrid coil 20. This does not preventthe amplified voice currents from passing through the amplifier AE to the lme L As this short'circuit is connected directly across the input side of the amplifier AW however, it does prevent voice currents or 130 echo currents incoming over the line La 'sors may be used atsuitable from passing through the amplifier .A-Wd to the star network.

Since all of the two-wire lines have these suppressors, no echoes return to the center and it is not necessary to provide equipment to prevent echoes going from the centcr over the two-wire lines. No suppressor is shown at the local set of the station F and none will be required unless the transmitter T, is so placed as to be disturbed by sounds from the loud speaker R If the echo suppressors at the central point, as above described, are not su'fficient to secure the desired freedom from echoes and equality of transmission loss between the local and distant stations, ,still other. two-wire suppresoints at any or all of the two-wire lines. I on the other hand, each two-wire line is supplied with one or more echo suppressors at intermediate points between its ends, the difliculty from echoes atthe central point may be so small that echo suppressors need not be provided thereat. The arrangement shown, however, accomplishes the with a minimum of echo suppressing apparatus, since only one-half of a complete suppressor equipment is required for each line.

An arrangement of connections involving the use of echo suppressors in connection with a multiple-way repeater is illustrated in Fig. 2. Here an amplifier A is provided for transmitting from the local line Lg or from the line L to the line L The output of this amplifier is connected through the usual hybrid coil 30 to the line L A similar amplifier A, is connected through a hybrid coil 31 to the line L, to take care 0 transmission from lines L, and L Likewise, a one-Way amplifier A is connected through the hybrid coil 32 to the line L to take care of transmission from the lines L1. and Lg. An input potentiometer 33 is bridged across the hybrid coil 32 and has an inputtransiormer 34 connected over the path P to the amplifier A a similar input transformer 35' being connected over the path P to amplifier A Similarly, a potentiometer 36 is bridged across the midpoints of the hybrid coil 30 and is associated with the input windings 37 and 38 leading thro 'h paths P fiers A and respectively. Likewise, potentiometer 39 is bridged across the midpoints of hybrid coil 31, and input coils 4:0 and 41 are associated therewith in paths P and P respectively, leading to the ammeans of this arrangement an independent transmission path is provided for each direction between anytransmission in line and each of, the otherlines, the paths being balanced and independent of each other. Byproperly adjusting the gains of desired result f mitted through the and P to amplithe potential interference between the paths might result each amplifier the over-all transmission between any two stations of the group may be made the same as that between any other pair of stations. In order to take care of the echo conditions, one-half of a complete suppressor is required for each' line connected at the common point, these units being conventionally indicated atS S and S The short circuiting contacts of each unit are connected to the bridged points of the corresponding hybrid coil so as to inter-v cept echoes returning from the line. Consequently, if the transmittermf a particular line is operating, the voice currents coming into the center point pass through the suitable repeater paths to the other lines, and the echoes reflected from the distant terminals of the other lines return over said lines toward the center points but are suppressed by the short-circuiting contacts upon arriving at the center point.

The input terminals of the suppressor units must be connected so that operation will occur when transmission is from the center into the line but not when it is in rection. It is therefore not possible to connect the suppressor directly across the output of the amplifier as it would not only be operated by the amplified currents but might be also operated by currents incoming from the line and passing through the hybrid coil to the output circuit of the amplifier. The input of the suppressor, unit is therefore connected between the first and second stage of the amplifier. The second stage tube thereby prevents transmission from the line back to the echo suppressor, but the echo suppressor will respond to currents trans- 1 amplifier path to the me.

By using two-stalge amplifiers and connecting the suppressor in the stages as indicated in Fig. 2 the-desired result may be accomplished of having the echo suppressor respond to transmission from the center out over the line but not operate for direct transmission from the distant station over the line to the center. While this arrangement involves the use of a two-stage amplifier such an amplifier would probably be required even if the echp suppressor units were not used because it is necessar to make the input transformers such. as 3 and 38 low ratio transformers where the secondaries of several transformers must be connected in series, as is the case with the multiple-way repeater. If the ratios of these transformers were made high for the purpose of stepping up which operates the amplifier,

because of the capacity between the windings of the input transformers. This capacity might also impair the gain frequency characteristic ofjthe repeater. If the transformers op ate perfectly no interference can the opposite d1- tit til

take lace between paths. For example trans miss1on through the transformer 34 and over the path P to the amplifier A would not be diverted through the path P and through the coils 40 and 41 bac through the path P and amplifier A to the line L,. This is for the reason that only potential changes appear on the secondary side of the transformer 34, and while these potential changes will operate the amplifier A they cannot produce induced electromoti-ve forces through the transformers 40 and 41 to operate the amplifier A If, however, the transformers [4.0 and 41 were? high ratio transformers so that the impedances would be lar e acrossthese transformers, a capacity ath ietween the windings of the transormer 34 would permit enough currentto flow in the path as distinguished from mere potentlal variations to cause an apprecia le disturbance. If currents were induced in this path through the transformer 34, obviously potentials might be induced through transformers 40 and 41 in the path P to operate the amplifier A thus causing interference.

With this arran'gement also other two-wi re suppressors may be used at suitable points at any or all of the two-wirelines where the echo suppressors at the central point are not sucient to obtain the desired freedom from echoes and equality of transmission loss between local and distant stations. Likewise, if each two-wire line is supplied with one or more echo suppressors at intermediate points between its ends the echo suppressors'at the central point may not be required. The arrangement shown accomplishes the result with a minimum of echo suppressing apparatus since only one-half of the complete echo 1suppressor equipment is required for each it will be obvious that the general principles herein disclosed may be embodied in many other organizations widely diii'erent from these illustrated without departing from the spirit of the invention as defined in the following claims. What is claimed is:

interconnecting arrangement for several signaling stations, comprising an interconnecting station, transmission circuits rfrom the several signaling stations to. said interconnecting stations, means at said interconnecting station to enable any line to transmit to all others, and meahs to prevent currents transmitted over said interconnecting station out over the lines and then refiected back towards the interconnecting point from being transmitted through sai mterconnecting means.

2. An interconnecting arrangement for several signaling stations, comprising an.

lines extending from interconnecting station,

stations to said Ineach of said signaling lines,

terconne'cting stations, means at said interconnecting station to enable any line to transmit to all of the other lines, and means to enable signals originating at the terminal of any line to be transmitted over said line to the interconnecting station for transmission to the other lines, said means being arranged to prevent currents transmitted from the interconnecting station over the lines and then reflected back, from being transmitted through said interconnecting station.

3. An interconnecting arrangement for several signaling stations, comprising an interconnecting station, lines extending from said signaling. stationsto said interconnecting station, means at said interconnecting station for interconnecting the lines so that any line may transmit to all of the others, said means including equipment for render.- ing the over-all transmission between any pair of stations substantially equal to the over-all transmissiontbetween any other pair of stations, and means to prevent currents transmitted through said interconnectin stations over the lines and then reflected back towards 'the' interconnecting station, from passing through said interconecting station. 7

4. An interconnecting arrangement for several signaling stations, comprising an interconnecting station, lines extending from each of said signaling stations to said inter- 1 connecting station, means at said inter connecting station to interconnect the lines so that any line may transmit to all of the others, said means including amplifying arrangements whereby the over-all transmission equivalent between any pair of interconseveral signaling stations, comprising aninterconnecting station, lines extending from each signaling station to the interconnecting station, a branched network at said in terconnecting station for interconnecting the the impedance of said not work lookinto any branch being equal to the iminto any other branch,

in pe ance looking amplifiers interposed between each line and the corre nding branch of the network to ren er the over-all transmission equivalent between any pair of stations substantially equal to the over-all transmission equivalent between any other pair of stations, and an echo suppressor associated with said amplifier, said echo suppressor being non-responsive to signaling currents originating at a signaling station and transmitted over the line through the amplifier to the branched network, but being responsive to signaling currents transmitted from f the branched network toward the signaling station to prevent said transmission line 10 from transmitting through the network.

In testimony whereof, we have signed our names to this specification this 22nd day of October, 1925.

. ALVA B. CLARK.

GEORGE CRISSON. 

